“A quarter-wave length wire, approximately 80 mm, will work very well as an antenna and the radiation minor affected if you place it parallel with the PCB, as long as you keep the distance to ground > 10 mm. If this is not possible in the design, keep at least the tip (last 4 cm of the wire) as far as possible from ground cause this is the most sensitive part of the antenna.
We have seen PCB designs where the antenna is routed parallel to the GND plane but with distance around 10 mm to ground, and range is close to the same as our demo board, which is close to optimum for a board with the antenna mounted on one side of the board (optimum would be a circular board of radius > 80 mm with the antenna in the middle).
So connecting a wire directly to the RF-pin of the module, and having a GND-plane in the PCB as big as possible (the bigger the better radiation), and grounding the modules’ GND-pins (directly) to this plane, will give you a good radiation, close to what is optimum. The orientation of the antenna (vertical or horizontal) is only possible to detect when the RX and TX antennas are close (near-field effects). when they are in some 20-30m distance the reflections are so many so that the orientation does not change the RSSI (received signal strength) value. In theory, there are no connection if you place two antennas with the tip pointing towards each other but this is also not possible to see in the far-field (long distance). Which is good as it gives more flexibility during installation 
Also keep the tip of the antenna away from plastic (some mm) as the plastic will also de-tune the antenna slightly.
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Last Update: November 16, 2017